Fix behavior of noidle in the presence of pending changes

According to the protocol specification, the output of the "noidle"
command is in principal the same as that of "idle", but might be empty.
Without this patch, python-mpd2 raises a ProtocolError if the response
to noidle is nonempty.
Update tests to match the changes in the last commit.
Merge pull request #32 from supermihi/master

Fix behavior of noidle in the presence of pending changes